SAW (surface acoustic wave) filters play a key role in telecommunications. For example, SAW filters are widely used as bandpass and spectrum-shaping filters in mobile and wireless applications. Other applications for SAW filters include wide area networks (WAN), wireless local area network (WLAN) communications, cordless phones, pagers and satellite communications. SAW filters are preferable to conventional LC filters as they are much smaller, cheaper and more versatile, making them ideal for telecommunication applications.
In SAW filters, electrical signals are converted to a mechanical wave in a device constructed of a piezoelectric crystal or ceramic. The wave is delayed as it propagates across the device, before being converted back to an electrical signal by other electrodes. More specifically, coupling between surface waves and electrical signals is achieved by interdigital transducers (IDTs). A simple form of the IDT consists of parallel fingers alternately connected to opposite electrodes to which the signal is applied.
For example, when an AC voltage is applied to input transducers, due to piezoelectricity, the transducer produces mechanical deformation of the piezoelectric substrate surface. This, in turn, results in a surface acoustic wave traveling on the surface of the piezoelectric substrate until reaching the output IDT, where it is transformed back to an electrical signal. When this wave reaches the output IDT, the electric field will induce a potential difference between neighboring electrodes such that the output IDT will convert the mechanical vibration into output voltages.
SAW filters can be designed to provide quite complex signal processing functions within a single package containing only a piezoelectric substrate with superimposed thin metal film input and output interdigital transducers (IDTs). SAW filters can be mass-produced using semiconductor microfabrication techniques, which enables the outstanding reproducibility of the SAW filters. However, programming or tuning of the SAW filters has been found to be difficult to achieve.
BAW (Bulk Acoustic Wave) filters are electromechanical devices, which typically operate at frequencies from around 2 GHz to around 16 GHz, and may be smaller or thinner than equivalent SAW filters. The BAW filter can be a Thin Film Bulk Acoustic Resonator (FBAR or TFBAR), which is a device consisting of a piezoelectric material sandwiched between two electrodes and acoustically isolated from the surrounding medium. FBAR devices using piezoelectric films can be used as radio frequency (RF) filters for use in cell phones and other wireless applications. FBARs can also be used in a variety of other applications such as microwave oscillators and sensor applications.
FBARs utilize the acoustic resonant characteristics of piezoelectric materials, much like SAW filters, to remove unwanted frequencies from being transmitted in a device, while allowing other specific frequencies to be received and transmitted. To adjust the acoustic resonant characteristics of the FBAR, thickness of the piezoelectric film can be adjusted. Alternatively or in addition, the FBAR can be put under a mechanical load so that its resonance frequency can shift. This is known as a mass loading effect. The mass loading effect can be accomplished by depositing or growing a mass of film on the resonator to bring about downshifting of the resonance frequency of the FBAR. The mass loading is carried out by growing or depositing of a thin film material uniformly distributed on one electrode of the resonator, covering the active surface of the device. The thin film can be a dielectric material or a metal material, depending on the design criteria. However, much like in the SAW application, programming or tuning of the BAW filter has been found to be difficult to achieve.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.